There are conventionally known power semiconductor devices in which an insulated gate bipolar transistor (IGBT) that uses a silicon (Si) substrate as a semiconducting material, and a freewheeling diode (FWD) that is connected in inverse-parallel with the IGBT are formed in the same semiconductor substrate.
For example, Patent Document 1 discloses a semiconductor device in which an IGBT and an FWD are formed in the same semiconductor substrate, a common electrode that functions as both an emitter electrode of the IGBT and an anode electrode of the FWD is formed on the front side of the substrate, and a common electrode that functions as both a collector electrode of the IGBT and a cathode electrode of the FWD is formed on the rear side of the substrate. Such a semiconductor device is called a reverse conducting (RC-) IGBT.
In general, the Si substrate has a long carrier lifetime, and therefore in the RC-IGBT, the recovery characteristics of the FWD often become a problem. Specifically, when the operating (forward conducting) state of the IGBT is switched to the reverse recovery (recovery) state of the FWD, a forward current continues to flow until minority carriers accumulated in the semiconductor substrate recombine and disappear, thus resulting in an increase in switching loss.
As a method for improving the recovery characteristics of the FWD, a carrier lifetime control method is used, in which a short carrier lifetime layer is formed by irradiating the entire semiconductor substrate, i.e., the entire IGBT region and the entire FWD region, with radiation such as light ions or electron beams and introducing crystal defects in the semiconductor substrate as carrier lifetime killers.
With this method, the crystal defects in the semiconductor substrate become centers of recombination, accelerating the recombination of minority carriers at the time of recovery of the FWD and improving the recovery characteristics of the FWD.
Patent Document 2 proposes a method in which a region for forming a short carrier lifetime layer is limited to only the FWD region (i.e., carrier lifetime killers are not introduced in the IGBT region), and discloses a technique for improving the recovery characteristics of the FWD without impairing the current-carrying capability of the IGBT.